how can stabilize the mosfet's drain current help (linear amplifier)

[tzitzikas1]

hi, i have this linear amplifier project http://tzitzikas.webs.com/irf640-1.JPG with 2 irf640 mosfets.
My problem is, when i adjust the both mosfets drain's current at 400mA (current without input signal), after some minutes

when the heatshink is warm (with input signal), the current becomes very high and one of the 2 fet goes off, and i must

replace it. Do you have any idea-schematic (with thermistor or diode) to control this current and to keep it stable at 400mA?

 

[audioguru2]

Each Mosfet dissipates (12V x 0.4A)= 4.8W. When a Mosfet gets hot then its current reduces and only increases when it melts. Maybe your heatsink is too small.

 

[Kevin Weddle]

Doesn't the top MOSFET receive a larger signal than the bottom MOSFET? They operate in parallel, but have inputs of different magnitude, and outputs of different magnitude. This means that the MOSFET's can be damaged easily if the circuit isn't built right with new components.